Internal Samsung SDI documents marked “Strictly Confidential” have leaked, and whoever authorised that distribution list is probably having a rough morning.

The files show Samsung testing smartphone batteries of 12,000 mAh and 18,000 mAh — while the Galaxy S26 Ultra, which went on sale recently, ships with 5,000 mAh.

Big numbers, messier reality

Two development branches show up in the documents. One targets 12,000 mAh, the other 18,000 mAh — nearly four times what the S26 Ultra ships with. Both run on silicon-carbon battery tech, which is what’s been allowing Chinese manufacturers to dramatically increase capacity without turning their phones into bricks.

On paper, either figure would make Samsung the runaway leader in battery capacity overnight.

Neither is going smoothly. The 12,000 mAh version is already coming out too thick in several prototypes, with swelling being monitored. The 18,000 mAh version runs hot in one of its cells and the charging management software isn’t finished.

And looming over both is the uncomfortable fact that Samsung’s previous 20,000 mAh prototype gave up at 960 charge cycles — well short of the 1,500-cycle target. A phone that degrades badly before year three is a hard sell at any price.

Late to a party Samsung once hosted

No product has been announced. No timeline exists. Right now these are lab experiments, and fairly troubled ones at that.

The irony is that Samsung has done big batteries before. The Galaxy M51 launched in India in 2020 for around $300 with a 7,000 mAh cell that genuinely lasted two days. Then the company just… stopped caring. The years kept passing, the flagships kept shipping with the same number, and nobody at Samsung seemed particularly bothered.

Over in the US, OnePlus launched the 15R with a 7,400 mAh battery for $699 — and that’s the largest you’ll find in any mainstream phone stateside right now. India and China moved even faster; phones there crossed 10,000 mAh while Samsung was apparently busy elsewhere.